There are currently many known arrangements and techniques for trapping or storing charged particles for the purposes of mass spectrometry. In some such arrangements, for example 3-D RF traps, linear multipole RF traps, and the more recently developed “Orbitrap”, ions injected into or formed within the trap oscillate within the trap with simple harmonic motion. In that case, ions may be selected for onward transmission to other traps, for mass analysis/detection, and so forth, by applying oscillating fields to the trap. This is because all of the ions of a given mass to charge ratio within the trap have a secular frequency of oscillation, such that ions of a specific mass to charge ratio may be resonantly excited out of the trap through application of a time-varying field to the whole of the trap.
In other multi-reflection systems, however, ions do not undergo simple harmonic motion. One example of such a trap is an electrostatic trap with two opposing reflectors. In such a trap, ions repeatedly traverse a space under the action of a field or fields and are reflected by at least two ion reflectors. In this type of trap, the application of an oscillating field will not select ions of just one mass to charge ratio. This is because ions of one mass to charge ratio are oscillating in the trap with a range of frequency components, not just one as they would if oscillating with simple harmonic motion. Whilst the ions of each mass to charge ratio have a unique period of oscillation, they do not oscillate with sinusoidal motion, and they can be excited by sinusoidal time varying fields which have a range of frequencies. Because of this, application of a single frequency sinusoidal excitation field to the trap will excite ions with a range of mass to charge ratios and cannot be used to select ions with high mass resolution.
Even though ions of different mass to charge ratios may have similar frequency components, they will, as noted above, nevertheless have a unique period of oscillation in the trap. In other words, ions of mass to charge ratio (m/z)1 will pass a notional point in the trap at times t1, t2, t3, t4 . . . , where (t2−t1)=(t3−t2)=(t4−t3) . . . whereas ions of a different species having mass to charge ratio (m/z)2 will pass the same point at times ta, tb, td . . . , where (tb−ta)=(tc−tb)=(td−tc) . . . but where (tb−ta) does not equal (t2−t1).
Therefore, by applying an excitation field to a specific localised part of the trap, at a particular time, ions of a given mass to charge ratio can be excited. Whilst it is possible to excite only the ions of interest (that is, only the ions having the desired mass to charge ratio m/z), in the practice normally the inverse of this is employed, and the excitation field is applied to all ions except those having the mass to charge ratio of interest, such that unwanted ions are excited out of the trap or so that they collide with a structure in the trap and are lost. Repeatedly turning the excitation field off, each time the ions of interest are in the excitation region, narrows the mass to charge ratio range of ions that are within the trap. Ions of a single, narrow, range of mass to charge ratios are selected in this way. The excitation field is usually generated by applying a voltage pulse to a deflector electrode which is positioned close to the ion path within the trap.
A typical prior art reflection trap employing such a principle is described in U.S. Pat. No. 3,226,543. Here, positive ions travel between two positively biased reflection electrodes forming a reflection trap. One of the reflection electrodes has the positive reflecting bias applied only when ions of a desired mass to charge ratio reach it, all other ions then passing through the de-energized reflector so that they are lost. A similar reflection trap is described in U.S. Pat. No. 6,013,913; opposing reflection electrodes are provided and one of these is unbiased during a particular time interval to allow desired ions to pass through the reflector and reach a detector. In U.S. Pat. No. 6,013,913, in order to improve transmission, an electrostatic particle guide is employed between the opposing reflectors. This guide also allows selective ejection of ions from the ion flight path.
Higher and higher mass to charge ratio resolution can be achieved using the repeated excitation techniques described above, provided only that the ions oscillate isochronously and can be held in the trap for sufficiently long periods of time. Both of these requirements are usually limited by ion optical imperfections of the trap, which set a limit on the useful time period—there is nothing further to be gained in continuing to oscillate the ions once the resolution limit of the trap has been reached. Additional oscillations simply expose the ions to further scattering events with background gas in the trap. Typically, the time limit is of the order of several, to several hundred milliseconds.
In some prior art systems, such as the one described in the above-referenced U.S. Pat. No. 6,888,130, the trap may optionally on occasion be operated at relatively low mass to charge resolution, and ions over a continuous but relatively large mass to charge ratio range are selected and ejected in one stage for further processing or detection.
Prior art methods of ion ejection suffer from a serious disadvantage, in that ions of only one mass to charge ratio (at high resolution), or ions of a continuous range of adjacent mass to charge ratios (at low resolution) are selected at a time. At high resolution, only one ion species can be selected during every fill of the trap, that is, only one ion species in each useful time-period may be analysed. For a single MS/MS experiment, in which a parent ion is to be selected, this might be all that is required. However, to acquire an extended mass spectrum at high resolution or multiple MS/MS experiments would require a great many trap fills, and a long elapsed time. If the sample material to be analysed is limited, it might be that only a small mass range could be analysed using this method. In the case of low resolution mass detection of a range of adjacent mass to charge ratios, there is an additional problem. In the next stage of processing or detection, the response time of a typical high dynamic range detector (formed by a charged particle multiplier detection system such as a channeltron or electron multiplier with an array of dynodes) is of the order of 1-10 microseconds. Specialized detectors for time-of-flight mass spectrometers are capable of shorter response times, although their dynamic range is typically much lower. This is caused by the fact that peak current in such detectors is comparable to that in slower, traditional detectors whilst the duration of the mass peak (and hence total charge detected) is much smaller. The typical pulse width of a packet of ions exiting the multi-reflection trap is of the order of 20-100 ns. This is several orders of magnitude shorter than the response time of typical detectors and thus limits resolution of ions of adjacent mass to charge ratios of significantly differing abundances.